A threaded engagement device has been widely used as a method of fastening two separate members. FIG. 8 shows the threaded engagement structure of a conventional threaded engagement device for fastening two members by threaded engagement. A threaded engagement device 100 is conventionally configured to fasten and secure a member with a female thread portion 110 and a member with a male thread portion 120 by engaging a plurality of threads formed in the female thread portion 110 and a plurality of threads formed in the male thread portion 120.
In a conventional threaded engagement device 100 such as that shown in FIG. 8, four thread surfaces of a plurality of threads formed in the female thread portion 110 and the male thread portion 120 (i.e., crest 111, 121, root 112, 122, stab flank 113, 123 and load flank 114, 124) are formed as a straight line. Further, the conventional threaded engagement device 100 is configured such that when the threaded engagement device 100 is engaged, the load flank 114 of the female thread portion 110 and the load flank 124 of the male thread portion 120 are contacted with each other, thereby fastening the threaded engagement device 100.
Such conventional threaded engagement device 100 is configured such that when the external force is applied in the opposite direction to the threaded engagement direction (in the direction “B” in FIG. 8), the load flank 114 of the female thread portion 110 and the load flank 124 of the male thread portion 120 receive the external force. Thus, in such conventional threaded engagement device 100, when the large external force is applied in the opposite direction to the threaded engagement direction (in the direction “B” in FIG. 8), significant stress is applied to the foot of the load flank 114 of the female thread portion 110 (“X” in FIG. 8) and the foot of the load flank 124 of the male thread portion 120 (“Y” in FIG. 8) to damage the threads of the female thread portion 110 and the male thread portion 120.
As shown in FIG. 8, the conventional threaded engagement device 100 is further configured such that the load flanks 114, 124 make a positive angle α with a straight line perpendicular to the threaded engagement axis, i.e., the load flank 114 makes an obtuse angle β with the crest 111 and the load flank 124 makes an obtuse angle β with the crest 121. In such a structure, the female thread portion 110 receives a force outward of the male thread portion 120 (in the direction “D” in FIG. 8) when the external force is applied in the direction for loosening the threaded engagement. Thus, such conventional threaded engagement device may be released easily by a relatively small external force applied in the direction for loosening the threaded engagement.